The present invention relates to an electrophotographic image forming apparatus and, more particularly, to an image forming apparatus of the type including a fixing unit made up of a heat roller and a press roller and a plurality of sheet feeding devices.
A copier, facsimile apparatus, printer or similar image forming apparatus of the type described is conventional. While sheets are sequentially fed from any one of a plurality of sheet feeding devices and conveyed via a fixing unit, the sheets absorb the heat of a heat roller included in the fixing unit and thereby lower the temperature of the roller. The degree of the temperature fall of the heat roller depends on, e.g., the sheet transport environment including the preset temperature and input voltage of the heat roller, the thickness, quality and other factors of sheets, and the interval between consecutive sheets. Specifically, assume that thick sheets are sequentially transported in a usual sheet interval mode assigned to usual sheets and setting up a sheet interval insuring desirable fixation even when the temperature of the heat roller falls. Then, the thick sheets noticeably absorbs the heat of the heat roller and thereby lowers the temperature of the heat roller to a critical degree.
In light of the above, it has been customary to increase the sheet interval for thick sheets in order to maintain fixation adequate. However, none of conventional implementations is capable of switching the sheet interval while a sequence of image forming cycles are under way. That is, the sheet interval can be switched only after the end of a sequence of image forming cycles. Assume that usual sheets are transported when the sheet interval greater than one assigned to usual papers is set. Then, the number of sheets that can be transported and therefore the image forming efficiency is reduced, and in addition power is wastefully consumed. Thus, should a sheet interval suitable for one of usual sheets and thick sheets be set up, the above trouble would occur during transport of the other sheets.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 63-184777, 4-319964, and 5-6043.